1. Field of the Invention
The present invention relates generally to directional diffusion films, polarizing plates, liquid crystal displays, and methods of manufacturing directional diffusion films.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) are used widely in, for example, PCs, automated teller machines (ATMs), and car navigation system equipment. Among LCDs, in a transmissive LCD with a twisted nematic (TN) mode employed therein, for example, viewing angle dependency is high, picture quality changes considerably when it is viewed from an oblique direction at a certain angle or more, and an image to be displayed in black under normal conditions appears whitish, and thus the contrast ratio may decrease or tone reversal may cause difficulty in viewing normal display. Therefore, a method has been proposed in which a light diffusion film is disposed on a front surface of a device for the purpose of securing the viewing angle of a LCD (for example, see JP 10(1998)-10513 A). According to this proposal, the tone reversal is suppressed and the viewing angle increases but there is a problem in that the contrast in the front direction decreases. Therefore, a directional light scattering film has been proposed that cancels the decrease in image quality while balancing a tone reversal prevention effect with contrast maintenance (for instance, see JP2006-133463 A). However, when such a light scattering film is disposed, the tone reversal was improved to some extent but with respect to the downward direction of the liquid crystal panel, it covers only a range of up to around 45°, and the contrast in the front direction of the liquid crystal panel also is improved but is still reduced by 20 to 30%. Accordingly, at present, it cannot be always said that the contrast is sufficient.
As described above, in the methods according to conventional light scattering techniques, the directional scattering properties (properties that the scattering properties vary when the incidence angle is changed) can be obtained only in a specific direction. Accordingly, light that is incident from a specific direction is scattered substantially uniformly when the effect of improving viewing angle dependency is obtained in the specific direction. Therefore a phenomenon occurs that display quality with respect to the other directions deteriorates. That is, the tone reversal suppression in the downward direction of the liquid crystal panel and the maintenance of the front contrast are in a trade-off relationship.